It's a known issue, when there are too many the deletion jams up. You can physically remove them from within the Windows environment.
First you have to enable viewing of hidden files and folders within Windows Explorer under Tools/Folder Options/View (Windows Explorer is any internal page such as Documents, Control Panel or Computer & not to be confused with its cousin Internet Explorer. If the Menu bar isn't showing in Windows Explorer go to Organize > Layout and check that item.)
Click image to enlarge....
First double-click the taskbar icon to open SecurityCenter
Click Navigation (top right)
Click General Settings & Alerts (left)
Click Access Protection to expand that section
Uncheck Access Protection and click Apply
Leave SecurityCenter open on your desktop because you should re-enable Access Protection after the following steps.
Go to C:\ProgramData\McAfee\VirusScan\Quarantine and click Edit/Select All
Click Shift and Delete simultaneously and the folder should empty.
Take care to delete only the contents of that folder, not the folder itself.
Re-enable Access Protection as mentioned above and click Apply and then exit SecurityCenter.
Those instructions are for Vista/Windows 7.
For XP the folder is found at: C:Documents & Settings/All Users/ApplicationData/McAfee/Virusscan
You can reduce their number in future considerably if you are prepared to accept Tracking Cookies. Those enable websites to remember your sign-in details etc.
With SecurityCenter open click Virus and Spyware Protection to expand that section.
Click Real-Time Scanning
Click Settings and scroll down to Tracking Cookies
Uncheck that and click Apply.
Don't forget to re-enable Access Protection.
The information provided by Ex_Brit is absolutely correct, but our suggestion is everytime if you run the McAfee scan prior to that run the McAfee Quick Clean.
Most of the Temporary files, Cookies are getting deleted by running the McAfee Quick Clean so if you run the McAfee Quick Clean everytime before the McAfee scan there are many possibilities the Tracking cookies will decrease in number.
Steps to Run McAfee Quick Clean :
Open McAfee Security Center - > Expand the Drawer PC and Home Network Tools - > Click on Quick Clean
The Below Snap gives the Clear picture of Quick Clean and the Options that are selected in the below snap is the default setting.
Click on Clean and Follow the Procedure. That should do it.